AU
B1666

This DTC indicates interrupted communication or a physical disconnection between the airbag control unit (SRS ECU) and the Left Side Impact Sensor — Seal U

Safety System

This DTC indicates interrupted communication or a physical disconnection between the airbag control unit (SRS ECU) and the Left Side Impact Sensor.

The Left Side Impact Sensor typically mounts in the lower left B-pillar or the left front seat side trim area.

It monitors left-side collision acceleration signals and provides key input for side airbag deployment decisions (including side airbags and side curtain airbags).

The ECU records B1666 if it fails to receive the sensor ID response code within the specified monitoring period, if LIN bus communication times out, or if it detects an open power or ground circuit.

This fault forces the left-side airbag system into fail-safe mode.

During a side impact, the airbags may fail to deploy or deploy late, severely compromising occupant side-impact protection.

Additionally, the instrument panel SRS warning lamp (airbag warning light) illuminates continuously, and the system may report the safety system fault status to the vehicle control unit via the CAN bus.

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Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Sensor harness connector loose, terminal backed out, or completely disconnected: Commonly occurs after accident repairs or interior trim removal/installation (such as replacing the left front door or B-pillar trim) due to failure to tighten the connector to the specified torque (usually 8-10 N·m), or a broken connector latch causing the connector to vibrate loose while driving.
  • 2Wiring harness physical damage or corrosion: Water ingress, underbody scraping, or rodent damage to the left sill wiring harness causes an open circuit or short to ground in the power supply line (+B), ground wire (GND), or LIN communication line. Connector pin oxidation creates high resistance (>5Ω).
  • 3Internal sensor body fault: damaged internal accelerometer element, cold solder joints on the PCB, aging sealing ring causing water ingress and internal short circuit, or sensor software version mismatch with the ECU (common when installing non-genuine replacement parts during accident repairs)
  • 4SRS ECU communication interface fault: Damaged ECU internal LIN transceiver, or damaged shielded wire between the ECU and sensor causing electromagnetic interference (EMI) and an excessively high communication bit error rate.
  • 5Deformed mounting bracket or poor ground connection: Failure to restore the sensor mounting base to standard after accident repairs causes a poor ground connection between the sensor and the vehicle body (ground resistance >1Ω), affecting the signal reference potential.
What To Do
  • 1
    Diagnostic Confirmation: Use a BYD VDS2000/3000 or Launch X-431 diagnostic tool to access the SRS system. Read all fault codes to confirm if B1666 is an Active or History code. Review the freeze frame data for vehicle status (vehicle speed, timestamp). Check for accompanying B1667 (left-side communication error) or U-series communication fault codes.
  • 2
    Safety preparation and visual inspection: Switch off the ignition, disconnect the negative battery terminal, and wait 90 seconds (to discharge the SRS capacitor). Remove the left B-pillar lower trim panel (trim clips are usually located beside the seat belt retractor cover). Verify the white crash sensor connector (part number usually starts with 5A-) seats fully with an audible locking 'click'. Inspect the connector for a missing waterproof sealing ring.
  • 3
    Electrical circuit measurement: Keep the ignition switch OFF, disconnect the sensor connector, and measure the harness-side terminals: ① Power supply terminal (usually pin 1, yellow wire) voltage to ground must be battery voltage (12-14V); ② Ground terminal (usually pin 3, brown wire) resistance to ground must be <1Ω; ③ LIN wire (usually pin 2, blue/black wire) resistance to ground must be 50-70Ω (terminating resistance). Use a multimeter continuity buzzer to check harness continuity and confirm no open circuits, short circuits, or high resistance.
  • 4
    Sensor verification: If the wiring is normal, perform a cross-check—swap the left and right sensors, clear the fault code, and test drive the vehicle. If the fault code changes to 'B1667 - Right Side Impact Sensor Disconnected', the sensor is faulty. Alternatively, measure the LIN line waveform directly using an oscilloscope. A normal signal displays a square wave with a 12V high level and 0V low level. If the sensor does not respond, the sensor is faulty.
  • 5
    Repair and calibration: Replace with genuine parts (match the part number to the VIN; some models require a sensor with a G mark). Seal wiring harness repairs with waterproof heat-shrink tubing. Sand the ground point to bare metal. Install the sensor to the specified torque (typically 8 N·m) with the arrow pointing toward the front of the vehicle. Reconnect the battery, clear the fault code, access 'Special Functions', and execute 'SRS System Self-Diagnosis' and 'Crash Sensor Zero-Point Calibration'. Finally, verify the repair with a tap test: lightly tap the B-pillar using a dedicated rubber hammer and observe the sensor acceleration value changes in the diagnostic tool data stream.
Real-World Cases
BYD DTC AI AnalysisFrom Chinese market (translated)

Unlocked connector after accident repair causes intermittent warning.

✓
A 2020 BYD Qin Pro EV displayed an intermittent "Check SRS System" warning on the instrument cluster after left-front collision repairs. A diagnostic scan showed current DTC B1666. Inspection found the left side impact sensor connector inserted but its locking tab not fully engaged, causing poor contact from road vibration. Reconnecting the connector until the secondary lock clicked and clearing the DTC resolved the issue. A one-week follow-up showed no recurrence. Fault resolved.
BYD DTC AI AnalysisFrom Chinese market (translated)

After driving through water, the wiring harness corroded, causing an open circuit in the sensor.

✓
2019 BYD Yuan EV: After wading through water during the rainy season, the airbag warning light stayed on. Retrieved DTC B1666, which would not clear. Disassembly revealed water ingress at the floor harness connector of the left front sill wiring harness. The yellow sensor power supply wire had corroded through. Cleaned oxidation from the harness connector, soldered the broken wire and applied waterproof insulation, and replaced the floor harness waterproof grommet. Fault resolved.
BYD DTC AI AnalysisFrom Chinese market (translated)

Aftermarket floor sound deadening installation pinched the wiring harness, causing a short circuit.

✓
The SRS warning light illuminated on a 2021 BYD Tang DM the day after the owner fitted full-coverage floor mats and floor sound deadening. Diagnosis revealed DTC B1666. Inspection found that when installing the sound deadening, the left lower B-pillar trim mounting screw pierced the side impact sensor wiring harness, causing a short to ground on the LIN line. Repaired the damaged harness (3 wires), rerouted the wiring to avoid contact with metal edges, resolved the fault, and reminded the owner of precautions when modifying the vehicle.
BYD DTC AI AnalysisFrom Chinese market (translated)

Damaged internal chip caused sensor communication failure.

✓
A 2019 BYD Song MAX with no accident history had the airbag warning light suddenly illuminate. Fault code B1666 was active. Measured wiring harness voltage, resistance, and LIN waveform – all normal. Swapped the left and right impact sensors and the fault moved to the right side, confirming an internal accelerometer chip fault in the sensor. Replaced the left impact sensor (part number 5A-3636400), performed the sensor calibration procedure, and the fault was resolved.
Other Seal U Fault Codes
B1108For the BYD Song MAX (2017–2019), DTC B1108 indicates a short circuit in the PM2.5 air quality detection module (dust sensor) or its wiring harness. The sensor uses laser scattering to detect in-cabin and outside PM2.5 concentrations and transmits data to the HVAC ECU via the LIN bus or an analog signal. Short circuit conditions include: 1) Sensor power supply wire (+5V/+12V) shorted to ground 2) Signal wire shorted to power or ground 3) Short circuit in the sensor's internal photoelectric detection circuit This fault prevents the automatic climate control from switching between fresh air and recirculation based on air quality. The system enters fail-safe mode (typically forced recirculation). Severe short-circuit current can destroy the HVAC ECU sampling circuit. The system classifies this as a Level 3 severe fault and restricts vehicle operation to prevent electrical fires or controller damage. For other BYD models (such as the Qin EV, E5, and Song DM), B1108 indicates a driver-side sunload sensor fault, reflecting differences in DTC definitions across models.B110811This DTC indicates a short circuit in the air conditioning system PM2.5 air quality sensor (rapid detector). Specifically, abnormal continuity exists between the sensor power supply circuit or signal output circuit and body ground or battery positive. This causes the control module (ACU/air conditioning controller) to detect abnormal current or a voltage signal outside the valid range (typically 0V or supply voltage). The PM2.5 sensor monitors internal and external particulate concentrations, providing the data the automatic air conditioning system uses to switch between fresh air and recirculation modes. A short circuit disables the air quality monitoring function. This fault may force the air conditioning system into recirculation protection mode and trigger related thermal management system faults, as the air conditioning system acts as a key actuator in vehicle thermal management. In severe cases, the short circuit current can damage the internal sampling circuit of the air conditioning controller or overheat the wiring harness.B1109DTC B1109 indicates an open circuit in the communication line between the air conditioning system PM2.5 air quality sensor (dust concentration sensor) and the air conditioning control unit (AC ECU), or an open circuit within the sensor internal circuitry. Typically located in the air conditioning intake duct, this sensor monitors real-time PM2.5 particle concentration inside and outside the vehicle. It provides data to the automatic air conditioning system for intelligent switching between fresh air and recirculation modes and for controlling the air purification device. During an open circuit fault, the AC ECU cannot receive the air quality signal, triggering the system fail-safe mode. Symptoms typically include failure of the automatic recirculation function, abnormal air purification indicator light operation, or abnormal air quality data displayed on the air conditioning panel. Severe cases may compromise the cabin environment control strategy, but do not directly affect vehicle driving safety.B110913DTC B110913 indicates an open signal circuit for the PM2.5 rapid detector (in-cabin air quality sensor). This sensor typically mounts inside the air conditioning system intake duct. It monitors in-cabin PM2.5 concentration in real time and provides data to the automatic air conditioning system for intelligent control of the air purification function. The BYD diagnostic protocol uses sub-code '13' to specifically indicate an open signal circuit. Although this fault theoretically affects a comfort feature, BYD Qin series vehicles classify it as a Level 3 (severe) fault. This classification occurs because the air conditioning system interacts with the battery thermal management system (e.g., monitoring battery cooling intake air quality). The fault can disable the automatic air conditioning mode and prevent the air purification function from operating. In extreme cases, it affects the battery compartment intake air quality assessment, triggering the 'repair immediately' strategy.B110AThis DTC indicates interrupted CAN bus communication or abnormal data between the air conditioning control unit (ACU) and the PM2.5 air quality sensor. The PM2.5 sensor monitors particulate concentration inside and outside the vehicle and provides the basis for purification control in the automatic air conditioning system. The ACU triggers this fault if it fails to receive a valid CAN message from the sensor within the specified period (usually 100-500 ms), if message verification fails, or if the signal frame times out. Although this fault does not affect vehicle driving safety, it disables the automatic air purification function. The air conditioning system cannot automatically switch between recirculation and fresh air modes or activate the air purification mode based on air quality. A persistent fault may restrict the thermal management strategy.
Data confidence: Official This information is for reference only. Always consult a qualified technician for diagnosis and repair. Do not attempt high-voltage system repairs yourself. Sources: [1]